JavaScript参考手册
Operators
typeof
typeof
操作符返回一个字符串,指示未经计算的操作数的类型。
语法
该typeof
操作是紧跟其操作:
typeof operand
参数
operand
是一个表达式,表示对象或原始值,其类型将被返回。
描述
下表总结了typeof可能的返回值。有关类型和原始值的更多信息,可查看 JavaScript数据结构 页面。
Type |
Result |
---|---|
Undefined |
"undefined" |
Null |
"object" (see below) |
Boolean |
"boolean" |
Number |
"number" |
String |
"string" |
Symbol (new in ECMAScript 2015) |
"symbol" |
Host object (provided by the JS environment) |
Implementation-dependent |
Function object (implements [Call] in ECMA-262 terms) |
"function" |
Any other object |
"object" |
示例
// Numbers
typeof 37 === 'number';
typeof 3.14 === 'number';
typeof(42) === 'number';
typeof Math.LN2 === 'number';
typeof Infinity === 'number';
typeof NaN === 'number'; // Despite being "Not-A-Number"
typeof Number(1) === 'number'; // but never use this form!
// Strings
typeof '' === 'string';
typeof 'bla' === 'string';
typeof '1' === 'string'; // note that a number within a string is still typeof string
typeof (typeof 1) === 'string'; // typeof always returns a string
typeof String('abc') === 'string'; // but never use this form!
// Booleans
typeof true === 'boolean';
typeof false === 'boolean';
typeof Boolean(true) === 'boolean'; // but never use this form!
// Symbols
typeof Symbol() === 'symbol'
typeof Symbol('foo') === 'symbol'
typeof Symbol.iterator === 'symbol'
// Undefined
typeof undefined === 'undefined';
typeof declaredButUndefinedVariable === 'undefined';
typeof undeclaredVariable === 'undefined';
// Objects
typeof {a: 1} === 'object';
// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === 'object';
typeof new Date() === 'object';
// The following is confusing. Don't use!
typeof new Boolean(true) === 'object';
typeof new Number(1) === 'object';
typeof new String('abc') === 'object';
// Functions
typeof function() {} === 'function';
typeof class C {} === 'function';
typeof Math.sin === 'function';
null
// This stands since the beginning of JavaScript
typeof null === 'object';
在 JavaScript 最初的实现中,JavaScript 中的值是由一个表示类型的标签和实际数据值表示的。对象的类型标签是 0。由于 null
代表的是空指针(大多数平台下值为 0x00),因此,null的类型标签也成为了 0,typeof null
就错误的返回了"object"
。(reference)
ECMAScript提出了一个修复(通过opt-in),但被拒绝。这将导致typeof null === 'object'。
使用 new
操作符
// All constructor function while instaitated with 'new' keyword will always be typeof 'object'
var str = new String('String');
var num = new Number(100);
typeof str; // It will return 'object'
typeof num; // It will return 'object'
//But there is a exception in case of Function constructor of Javascript
var func = new Function();
typeof func; // It will return 'function'
正则表达式
对正则表达式字面量的类型判断在某些浏览器中不符合标准:
typeof /s/ === 'function'; // Chrome 1-12 Non-conform to ECMAScript 5.1
typeof /s/ === 'object'; // Firefox 5+ Conform to ECMAScript 5.1
例外
所有当前的浏览器都暴露了一个类型为 undefined 的非标准宿主对象 document.all。
typeof document.all === 'undefined';
尽管规范允许为非标准的外来对象定制类型标签,但它要求这些类型标签与预定义标签不同。document.all
的类型标记为“undefined
”的情况必须被列为违反规则的特殊情况。
规范
Specification |
Status |
Comment |
---|---|---|
ECMAScript Latest Draft (ECMA-262)The definition of 'The typeof Operator' in that specification. |
Living Standard |
|
ECMAScript 2015 (6th Edition, ECMA-262)The definition of 'The typeof Operator' in that specification. |
Standard |
|
ECMAScript 5.1 (ECMA-262)The definition of 'The typeof Operator' in that specification. |
Standard |
|
ECMAScript 3rd Edition (ECMA-262)The definition of 'The typeof Operator' in that specification. |
Standard |
|
ECMAScript 1st Edition (ECMA-262)The definition of 'The typeof Operator' in that specification. |
Standard |
Initial definition. Implemented in JavaScript 1.1. |
浏览器兼容性
Feature |
Chrome |
Edge |
Firefox (Gecko) |
Internet Explorer |
Opera |
Safari |
---|---|---|---|---|---|---|
Basic support |
(Yes) |
(Yes) |
(Yes) |
(Yes) |
(Yes) |
(Yes) |
Feature |
Android |
Chrome for Android |
Edge |
Firefox Mobile (Gecko) |
IE Mobile |
Opera Mobile |
Safari Mobile |
---|---|---|---|---|---|---|---|
Basic support |
(Yes) |
(Yes) |
(Yes) |
(Yes) |
(Yes) |
(Yes) |
(Yes) |
Operators相关
JavaScript 是一种高级编程语言,通过解释执行,是一门动态类型,面向对象(基于原型)的解释型语言。它已经由ECMA(欧洲电脑制造商协会)通过 ECMAScript 实现语言的标准化。它被世界上的绝大多数网站所使用,也被世界主流浏览器( Chrome、IE、FireFox、Safari、Opera )支持。JavaScript 是一门基于原型、函数先行的语言,是一门多范式的语言,它支持面向对象编程,命令式编程,以及函数式编程。它提供语法来操控文本、数组、日期以及正则表达式等,不支持 I/O,比如网络